Little is known concerning the comparative anatomy of motor pathways in the various representatives of living mammals. This is particularly true for those cortical and subcortical systems traditionally considered to be extrapyramidal. The purpose of this study is to extend the results of previous investigations on morphological and functional details of such pathways in generalized marsupial and placental mammals, as well as in certain more specialized representatives of these forms. It is felt that such information will provide insight concerning the correlation between anatomical variations in these systems and differences in motor performance. Due to the anatomy of its corticospinal tract, the opossum provides a unique opportunity not available in more specialized forms to study the results of lesions within certain subcorticospinal systems. The results of such lesions will be studied in long term chronic experiments.